A reader contributes

A primary building block in BEAM robotics (
ref.
and
second ref.
) is the
Solar Engine, a very
simple two transistor circuit originally designed by
Mark Tilden,
which allows energy to be stored in a capacitor, until the charge
is sufficient to briefly run a tiny motor. This means you can
easily make a solar powered mechanical device `live' with just a
small solar panel in far less than direct sunlight - as long as you
don't need continuous movement. Powered by a Solar Engine, your
device will be active in short bursts, with pauses in between. The
length of a pause is determined by the available solar current.
The original Solar Engine has the advantage of extreme simplicity,
but also a few disadvantages. One of them is the way it achieves
reset. This happens only when the load almost completely
discharges the capacitor. Depending on your type of device, this
may waste energy because the motor stops long before the capacitor
is fully discharged, or reset may not be achieved at all, because
the device is incapable of discharging the capacitor to a
sufficiently low level.
I used a circuit much like the Solar Engine to switch the Photovore
on and off, the most important change being a reset independent of
load. The Canadian BEAMer Ken Huntington took my circuit and
changed it into a more efficient Solar Engine. Below you find his
diagram and his description of it. He also supplied some
oscilloscope images of the circuit
in operation. Enjoy!

Micro Power Solar Engine

by Ken (K.G.A.) Huntington

A 'micro power solar engine' has been a goal since my introduction to
BEAM Robotics. I believe that if there wasn't one before, I there is one
now. What I'm presenting to you looks very similar to one of the circuits
found in Steven Bolt's web pages. As you will see, I made only minor
changes to that design and not without help. My contributions have been
to recognize the potential of the circuit, bread board and test it. The
bottom line is that if you have a power source which will provide 2.5Vdc
at 10uA this circuit should drive a pager motor. It turns on the motor
at 2.3 to 2.5Vdc and switches off at 1.2 to 1.5Vdc.

Both Steve and I have built (bread boarded or haywired) the business end
of the design and achieved similar results. This does not mean that it
will work for everyone but it should.
[I used a BC559C (Q1), a BC549C (Q2) a BC559B (Q3) and a BC337-25
(Q4), which happened to be in the bitbox. SB]
There is one adjustable resistor which is the only real complication and
it means that there is a calibration procedure which is as follows:
1) Set R1 to max. (1.0M).
2) Connect a DVM across the storage capacitor.
3) Enable power to the circuit.
If the circuit works properly, the voltage will rise to a firing point,
turn on the motor momentarily which discharges the capacitor to level
where motor has little current passing through it but Q1 and Q2 are
still conducting. At this point,...
4) Slowly adjust R1 until the voltage starts to rise again.
This is the threshold for minimum operation.
5) Adjust R1 just a little further to assure reliable operation.
In practice, I removed R1, measured its value and replaced it with a
standard resistor value 5 to 10% less. 560K was the value that both
Steve and I used which shows just how repeatable this design is or how
lucky we were.
Note: It is realized that this presentation is very cryptic. If it were
delayed until a complete circuit description and circuit layout were
complete, it may not have been presented at all. It is hard for many to
realize the time and effort it takes to create the circuit cards, obtain
the correct parts, make up the kits and write assembly/debug
instructions. The dollar cost and other risks are not minimal.
If there is a need for this circuit, it shall come to pass.
KG

A layout has recently been published by Eric Platt, on
this page,
which also shows a nice photo of the very compact result.

Since Ken wrote this, I dabbled a bit in Solar Engines myself. This
resulted in two "SunEaters". They found a home on
this page edited by Brian O. Bush.